Maintenance Engineering -- ALL ABOUT BEARINGS





A bearing is a machine element that supports a part-such as a shaft-that rotates, slides, or oscillates in or on it. There are two broad classifications of bearings, plain and rolling element (also called anti-friction). Plain bearings are based on sliding motion made possible through the use of a lubricant. Anti-friction bearings are based on rolling motion, which is made possible by balls or other types of rollers. In modern rotor systems operating at relatively high speeds and loads, the proper selection and design of the bearings and bearing-support structure are key factors affecting system life.

TYPES OF MOVEMENT

The type of bearing used in a particular application is determined by the nature of the relative movement and other application constraints. Movement can be grouped into the following categories: rotation about a point, rotation about a line, translation along a line, rotation in a plane, and translation in a plane.

These movements can be either continuous or oscillating.

Although many bearings perform more than one function, they can generally be classified based on types of movement, and there are three major classifications of both plain and rolling element bearings: radial, thrust, and guide. Radial bearings support loads that act radially and at right angles to the shaft center line. These loads may be visualized as radiating into or away from a center point like the spokes on a bicycle wheel. Thrust bearings support or resist loads that act axially. These may be described as endwise loads that act parallel to the center line towards the ends of the shaft. This type of bearing prevents lengthwise or axial motion of a rotating shaft.

Guide bearings support and align members having sliding or reciprocating motion. This type of bearing guides a machine element in its lengthwise motion, usually without rotation of the element.

Tbl. 0-9.1 gives examples of bearings that are suitable for continuous movement;

Tbl. 0-9.2 shows bearings that are appropriate for oscillatory movement only.

For the bearings that allow movements in addition to the one listed, the effect on machine design is described in the column, "Effect of the Other Degrees of Freedom." Tbl. 0-9.3 compares the characteristics, advantages, and disadvantages of plain and rolling element bearings.

ABOUT A POINT (ROTATIONAL)

Continuous movement about a point is rotation, a motion that requires repeated use of accurate surfaces. If the motion is oscillatory rather than continuous, some additional arrangements must be made in which the geometric layout prevents continuous rotation.

ABOUT A LINE (ROTATIONAL)

Continuous movement about a line is also referred to as rotatio n, and the same comments apply as for movement about a point.

ALONG A LINE (TRANSLATIONAL)

Movement along a line is referred to as translatio n. One surface is generally long and continuous, and the moving component is usually supported on a fluid film or rolling contact to achieve an acceptable wear rate. If the translational movement is reciprocation, the application makes repeated use of accurate surfaces, and a variety of economical bearing mechanisms are available.

IN A PLANE (ROTATIONAL / TRANSLATIONAL)

If the movement in a plane is rotational or both rotational and oscillatory, the same comments apply as for movement about a point. If the movement in a plane is translational or both translational and oscillatory, the same comments apply as for movement along a line.

COMMONLY USED BEARING TYPES

As mentioned before, major bearing classifications are plain and rolling element.

These types of bearings are discussed in the sections to follow. Tbl. 0-9.4 is a bearings characteristics summary. Tbl. 0-9.5 is a selection guide for bearings operating with continuous rotation and special environmental conditions.

Tbl. 0-9.6 is a selection guide for bearings operating with continuous rotation and special performance requirements. Tbl. 0-9.7 is a selection guide for oscillating movement and special environment or performance requirements.

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Tbl. 0-9.1 Bearing Selection Guide (Continuous Movement)

Constraint applied to the movement

About a point About a line Along a line Crane wheel restrained between two rails Railway or crane wheel on a track Pulley wheel on a cable Hovercraft or hoverpad on a track Single thrust bearing Single thrust bearing must be loaded into contact Double thrust bearing In a plane (rotation) In a plane (translation) Hovercraft or hoverpad Needs to be loaded into contact usually by gravity These arrangements need to be loaded into contact. This is usually done by gravity.

Wheels on a single rail or cable need restraint to pre vent rotation about the track member Gimbals Journal bearing with double thrust location Journal bearing Screw and nut Ball joint bearing or spherical roller Allows some angular freedom to the line of rotation Double conical bearing Simple journal bearing allows free axial movement as well Gives some related axial movement as well Ball on a recessed plate Ball must be forced into contact with the plate

Examples of arrangements which allow movement only within this constraint Examples of arrangements which allow this movement but also have other degrees of freedom Effect of the other degrees of freedom

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Tbl. 0-9.2 Bearing Selection Guide (Oscillatory Movement)

Constraint applied to the movement; Examples of arrangements which allow movement only within this constraint; Examples of arrangements which allow this movement but allows; have other degrees of freedom; Effect of the other degrees of freedom

About a point About a line Along a line In a plane (rotation) In a plane (translation) Plate between upper and lower guide blocks Rubber ring or disc Block sliding on a plate Must be loaded into contact; Gives some axial and lateral flexibility as well Crossed strip flexed pivot; Torsion suspension Knife-edge pivot Rubber bush Gives some axial and lateral flexibility as well Rocker pad Gives some related translation as well. Must be loaded into contact Piston and cylinder Crosshead and guide bars Piston can rotate as well unless it's located by connecting rod; Must be loaded into contact A single torsion suspension gives no lateral location Hookes joint Cable connection between components Cable needs to be kept in tension

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Tbl. 0-9.3 Comparison of Plain and Rolling Element Bearings

Rolling Element Plain

Assembly on crankshaft is virtually impossible, except with very short or built-up crankshafts Assembly on crankshaft is no problem as split bearings can be used Cost relatively high Cost relatively low Hardness of shaft unimportant Hardness of shaft important with harder bearings Heavier than plain bearings Lighter than rolling element bearings Housing requirement not critical Rigidity and clamping most important housing requirement Less rigid than plain bearings More rigid than rolling element bearings Life limited by material fatigue Life not generally limited by material fatigue Lower friction results in lower power consumption Higher friction causes more power consumption Lubrication easy to accomplish, the required flow is low except at high speed Lubrication pressure feed critically important, required flow is large, susceptible to damage by contaminants and interrupted lubricant flow Noisy operation Quiet operation Poor tolerance of shaft deflection Moderate tolerance of shaft deflection Poor tolerance of hard dirt particles Moderate tolerance of dirt particles, depending on hardness of bearing Requires more overall space: Requires less overall space: Length: Smaller than plain Length: Larger than rolling element Diameter: Larger than plain Diameter: Smaller than rolling element Running Friction: Running Friction: Very low at low speeds Higher at low speeds May be high at high speeds Moderate at usual crank speeds Smaller radial clearance than plain Larger radial clearance than rolling element

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Tbl. 0-9.4 Bearing Characteristic Summary

Bearing Type Description

Plain See Tbl. 0-9.3 Lobed See Radial, Elliptical Radial or journal Cylindrical Gas lubricated, low-speed applications Elliptical Oil lubricated, gear and turbine applications, stiffer and somewhat more stable bearing Four-axial grooved Oil lubricated, higher-speed applications than cylindrical Partial arc Not a bearing type, but a theoretical component of grooved and lobed bearing configurations Tilting pad High-speed applications where hydrodynamic instability and misalignment are common problems Thrust Semi-fluid lubrication state, relatively high friction, lower service pressures with multi-collar version, used at low speeds Rolling element See Tbl. 0-9.3. Radial and axial loads, moderate- to high-speed applications Ball Higher speed and lighter load applications than roller bearings Single-row Radial non-filling slot Also referred to as Conrad or deep-groove bearing; sustains combined radial and thrust loads, or thrust loads alone, in either direction, even at high speeds; not self-aligning Radial filling slot Handles heavier loads than non-filling slot Angular contact radial thrust Radial loads combined with thrust loads, or heavy thrust loads alone; axial deflection must be limited Ball-thrust Very high thrust loads in one direction only, no radial loading, can't be operated at high speeds Double-row Heavy radial with minimal bearing deflection and light thrust loads Double-roll, self-aligning Moderate radial and limited thrust loads Roller Handles heavier loads and shock better than ball bearings, but are more limited in speed than ball bearings Cylindrical Heavy radial loads, fairly high speeds, can allow free axial shaft movement Needle-type cylindrical or barrel Does not normally support thrust loads, used in space-limited applications, angular mounting of rolls in double-row version tolerates combined axial and thrust loads Spherical High radial and moderate-to-heavy thrust loads, usually comes in double-row mounting that's inherently self-aligning Tapered Heavy radial and thrust loads; can be preloaded for maximum system rigidity

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Tbl. 0-9.5 Bearing Selection Guide for Special Environmental Conditions (Continuous Rotation); Bearing Type High Temp.; Low Temp. Vacuum We t; Humid Dir t

Dust External Vibration

Plain, externally pressurized 1 (With gas lubrication) 2 No (Affected by lubricant feed) 2 2 (1 when gas lubricated) 1 Plain, porous metal (oil impregnated) 4 (Lubricant oxidizes) 3 (May have high starting torque) Possible with special lubricant 2 Seals essential 2 Plain, rubbing (non metallic) 2 (Up to temp. limit of material) 2 12 (Shaft must not corrode) 2 (Seals help) 2 Plain, fluid film 2 (Up to temp. limit of lubricant) 2 (May have high starting torque) Possible with special lubricant 2 2 (With seals and filtration) 2 Rolling Consult manufacturer above 15 0 8C 2 3 (With special lubricant) 3 (With seals) Sealing essential 3 (Consult manufacturers) Things to watch with all Effect of thermal expansion on fits Effect of thermal expansion on fits Corrosion Fretting Rating: 1-Excellent, 2-Good, 3-Fair, 4-Poor

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Tbl. 0-9.6 Bearing Selection Guide for Particular Performance Requirements (Continuous Rotation)

Bearing Type Accurate Radial Location Axial Load Capacity As Well Low Starting Torque Silent Running Standard Parts Available Simple Lubrication

Plain, externally pressurized 1 No (Need separate thrust bearing) 1 1 No 4 (Need special system) Plain, fluid film 3 No (Need separate thrust bearing) 2 1 Some 2 (Usually requires circulation system) Plain, porous metal (oil impregnated) 2 Some 2 1 Yes 1 Plain, rubbing (non-metallic) 4 Some in most instances 4 3 Some 1 Rolling 2 Yes in most instances 1 Usually satisfactory Yes 2 (When grease lubricated) Rating: 1-Excellent/very good, 2-Good, 3-Fair, 4-Poor

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Tbl. 0-9.7 Bearing Selection Guide for Special Environments or Performance (Oscillating Movement)

Bearing Type High Temp.; Low Temp.; Low Friction We t; Humid Dir t; Dust External Vibration

Knife edge pivots 2 212 (Watch corrosion) 2 4 Plain, porous metal (oil impregnated) 4 (Lubricant oxidizes) 3 (Friction can be high) 2 2 Sealing essential 2 Plain, rubbing 2 (Up to temp.

limit of material) 1 2 (With PTFE) 2 (Shaft must not corrode) 2 (Sealing helps) 1 Rolling Consult manufacturer above 15 0 8C 2 12 (With seals) Sealing essential 4 Rubber bushes 4 4 Elastically stiff 1 1 1 Strip flexures 2 1 1 2 (Watch corrosion) 1 1 Rating: 1-Excellent, 2-Good, 3-Fair, 4-Poor

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PLAIN BEARINGS

All plain bearings also are referred to as fluid-film bearing s. In addition, radial plain bearings also are commonly referred to as journal bearing s. Plain bearings are available in a wide variety of types or styles and may be self-contained units or built into a machine assembly. Tbl. 0-9.8 is a selection guide for radial and thrust plain bearings.

Plain bearings are dependent on maintaining an adequate lubricant film to prevent the bearing and shaft surfaces from coming into contact, which is necessary to prevent premature bearing failure.

However, this is difficult to achieve, and some contact usually occurs during operation. Material selection plays a critical role in the amount of friction and the resulting seizure and wear that occurs with surface contact. Refer to Section 3 for a discussion of common bearing materials. Note that fluid-film bearings don't have the ability to carry the full load of the rotor assembly at any speed and must have turning gear to support the rotor's weight at low speeds.

Thrust or Fixed

Thrust plain bearings consist of fixed shaft shoulders or collars that rest against flat bearing rings. The lubrication state may be semi-fluid, and friction is relatively high. In multi-collar thrust bearings, allowable service pressures are considerably lower because of the difficulty in distributing the load evenly between several collars. However, thrust ring performance can be improved by introducing tapered grooves. Ill. 0-9.1 shows a mounting half section for a vertical thrust bearing.

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Tbl. 0-9.8 Plain Bearing Selection Guide

Journal Bearings - Characteristics - Direct Lined - Insert Liners

Accuracy Dependent on facilities and skill available Precision components Quality (Consistency) Doubtful Consistent Cost Initial cost may be lower Initial cost may be higher Ease of Repair Difficult and costly Easily done by replacement Condition upon extensive use Likely to be weak in fatigue Ability to sustain higher peak loads Materials used Limited to white metals Extensive range available

Thrust Bearings Characteristic Flanged Journal Bearings Separate Thrust Washer

Cost Costly to manufacture Much lower initial cost Replacement Involves whole journal/ thrust component Easily replaced without moving journal bearing Materials used Thrust face materials limited in larger sizes Extensive range available Benefits Aids assembly on a production line Aligns itself with the housing

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Radial or Journal

Plain radial, or journal, bearings also are referred to as sleeve or Babbit bearings.

The most common type is the full journal bearing, which has 360-degree contact with its mating journal. The partial journal bearing has less than 180-degree contact and is used when the load direction is constant. The sections to follow describe the major types of fluid-film journal bearings: plain cylindrical, four-axial groove, elliptical, partial arc, and tilting-pad.

Plain Cylindrical

The plain cylindrical journal bearing ( Ill. 0-9.2) is the simplest of all journal bearing types. The performance characteristics of cylindrical bearings are well established, and extensive design information is available. Practically, use of the unmodified cylindrical bearing is generally limited to gas-lubricated bearings and low-speed machinery.

Four-Axial Groove Bearing

To make the plain cylindrical bearing practical for oil or other liquid lubricants, it's necessary to modify it by the addition of grooves or holes through which the lubricant can be introduced. Sometimes, a single circumferential groove in the middle of the bearing is used. In other cases, one or more axial grooves are provided.

The four-axial groove bearing is the most commonly used oil-lubricated sleeve bearing. The oil is supplied at a nominal gage pressure that ensures an adequate oil flow and some cooling capability. Ill. 0-9.3 illustrates this type of bearing.

Ill. 0-9.1 Half section of mounting for vertical thrust bearing.

Ill. 0-9.2 Plain cylindrical bearing.

Ill. 0-9.3 Four-axial groove bearing.

Ill. 0-9.4 Elliptical bearing.

Elliptical Bearing

The elliptical bearing is oil-lubricated and typically is used in gear and turbine applications. It is classified as a lobed bearing in contrast to a grooved bearing.

Where the grooved bearing consists of a number of partial arcs with a common center, the lobed bearing is made up of partial arcs whose centers don't coincide. The elliptical bearing consists of two partial arcs in which the bottom arc has its center a distance above the bearing center. This arrangement has the effect of preloading the bearing, where the journal center eccentricity with respect to the loaded arc is increased and never becomes zero. This results in the bearing being stiffened, somewhat improving its stability. An elliptical bearing is shown in Ill. 0-9.4.

Partial-Arc Bearings

A partial-arc bearing is not a separate type of bearing. Instead, it refers to a variation of previously discussed bearings (e.g., grooved and lobed bearings) that incorporates partial arcs. It is necessary to use partial-arc bearing data to incorporate partial arcs in a variety of grooved and lobed bearing configurations.

In all cases, the lubricant is a liquid and the bearing film is laminar. Ill. 0-9.5 illustrates a typical partial-arc bearing.

Tilting-Pad Bearings

Tilting-pad bearings are widely used in high-speed applications in which hydro-dynamic instability and misalignment are common problems. This bearing consists of a number of shoes mounted on pivots, with each shoe being a partial-arc bearing. The shoes adjust and follow the motions of the journal, ensuring inherent stability if the inertia of the shoes does not interfere with the adjustment ability of the bearing. The load direction may either pass between the two bottom shoes or it may pass through the pivot of the bottom shoe. The lubricant is incompressible (i.e., liquid) and the lubricant film is laminar. Ill. 0-9.6 illustrates a tilting-pad bearing.

Ill. 0-9.5 Partial-arc bearing.

Rolling Element or Anti-Friction

Rolling element anti-friction bearings are one of the most common types used in machinery. Anti-friction bearings are based on rolling motion as opposed to the sliding motion of plain bearings. The use of rolling elements between rotating and stationary surfaces reduces the friction to a fraction of that resulting with the use of plain bearings. Use of rolling element bearings is determined by many factors, including load, speed, misalignment sensitivity, space limitations, and desire for precise shaft positioning. They support both radial and axial loads and are generally used in moderate- to high-speed applications.

Unlike fluid-film plain bearings, rolling element bearings have the added ability to carry the full load of the rotor assembly at any speed. Where fluid-film bearings must have turning gear to support the rotor's weight at low speeds, rolling element bearings can maintain the proper shaft centerline through the entire speed range of the machine.

Ill. 0-9.6 Tilting-pad bearing.

Grade Classifications

Rolling element bearings are available in either commercial- or precision-grade classifications. Most commercial-grade bearings are made to non-specific standards and are not manufactured to the same precise standards as precision-grade bearings. This limits the speeds at which they can operate efficiently, and given brand bearings may or may not be interchangeable.

Precision bearings are used extensively in many machines such as pumps, air compressors, gear drives, electric motors, and gas turbines. The shape of the rolling elements determines the use of the bearing in machinery. Because of standardization in bearing envelope dimensions, precision bearings were once considered to be interchangeable, even if manufactured by different companies.

It has been discovered, however, that interchanging bearings is a major cause of machinery failure and should be done with extreme caution.

Ill. 0-9.7 Guide to selecting ball or roller bearings.

Rolling Element Types

There are two major classifications of rolling elements: ball and roller. Ball bearings function on point contact and are suited for higher speeds and lighter loads than roller bearings. Roller element bearings function on line contact and generally are more expensive than ball bearings, except for the larger sizes. Roller bearings carry heavy loads and handle shock more satisfactorily than ball bearings but are more limited in speed. Ill. 0-9.7 provides general guidelines to determine if a ball or roller bearing should be selected. This figure is based on a rated life of 30,000 hours.

Although there are many types of rolling elements, each bearing design is based on a series of hardened rolling elements sandwiched between hardened inner and outer rings. The rings provide continuous tracks or races for the rollers or balls to roll in. Each ball or roller is separated from its neighbor by a separator cage or retainer, which properly spaces the rolling elements around the track and guides them through the load zone. Bearing size is usually given in terms of boundary dimensions: outside diameter, bore, and width.

Ill. 0-9.8 Three principal types of ball bearing loads.

Ill. 0-9.9 Single-row radial, non-filling slot bearing.

Ball Bearings

Common functional groupings of ball bearings are radial, thrust, and angular-contact bearings. Radial bearings carry a load in a direction perpendicular to the axis of rotation. Thrust bearings carry only thrust loads, a force parallel to the axis of rotation tending to cause endwise motion of the shaft. Angular-contact bearings support combined radial and thrust loads. These loads are illustrated in Ill. 0-9.8. Another common classification of ball bearings is single row (also referred to as Conrad or deep-groove bearing) and double row.

Single-Row Types of single-row ball bearings are: radial non-filling slot bearings, radial filling slot bearings, angular contact bearings, and ball thrust bearings.

Radial, Non-Filling Slot Bearings This ball bearing is often referred to as the Conrad-type or deep-groove bearing and is the most widely used of all ball bearings (and probably of all anti-friction bearings). It is available in many variations, with single or double shields or seals. They sustain combined radial and thrust loads, or thrust loads alone, in either direction-even at extremely high speeds. This bearing is not designed to be self-aligning; therefore, it's imperative that the shaft and the housing bore be accurately aligned.

Ill. 0-9.10 labels the parts of the Conrad anti-friction ball bearing. This design is widely used and is versatile because the deep-grooved raceways permit the rotating balls to rapidly adjust to radial and thrust loadings, or a combination of these loadings.

Ill. 0-9.10 Conrad anti-friction ball bearing parts.

Radial, Filling Slot Bearing The geometry of this ball bearing is similar to the Conrad bearing, except for the filling slot. This slot allows more balls in the complement and thus can carry heavier radial loads. The bearing is assembled with as many balls that fit in the gap created by eccentrically displacing the inner ring. The balls are evenly spaced by a slight spreading of the rings and heat expansion of the outer ring. However, because of the filling slot, the thrust capacity in both directions is reduced. In combination with radial loads, this bearing design accommodates thrust of less than 60% of the radial load.

Angular Contact Radial Thrust This ball bearing is designed to support radial loads combined with thrust loads, or heavy thrust loads (depending on the contact-angle magnitude). The outer ring is designed with one shoulder higher than the other, which allows it to accommodate thrust loads. The shoulder on the other side of the ring is just high enough to prevent the bearing from separating. This type of bearing is used for pure thrust load in one direction and is applied either in opposed pairs (duplex) or one at each end of the shaft.

They can be mounted either face-to-face or back-to-back and in tandem for constant thrust in one direction. This bearing is designed for combination loads in which the thrust component is greater than the capacity of single-row, deep-groove bearings. Axial deflection must be confined to very close tolerances.

Ball-Thrust Bearing The ball-thrust bearing supports very high thrust loads in one direction only, but supports no radial loading. To operate successfully, this type of bearing must be at least moderately thrust-loaded at all times. It should not be operated at high speeds, since centrifugal force causes excessive loading of the outer edges of the races.

Ill. 0-9.11 Double-row type ball bearing.

Double-Row Double-row ball bearings accommodate heavy radial and light thrust loads without increasing the outer diameter of the bearing. However, this type of bearing is approximately 60-80% wider than a comparable single-row bearing. The double-row bearing incorporates a filling slot, which requires the thrust load to be light. Ill. 0-9.11 shows a double-row type ball bearing.

This unit is, in effect, two single-row angular contact bearings built as a unit with the internal fit between balls and raceway fixed during assembly. As a result, fit and internal stiffness are not dependent on mounting methods. These bearings usually have a known amount of internal preload, or compression, built in for maximum resistance to deflection under combined loads with thrust from either direction. As a result of this compression prior to external loading, the bearings are very effective for radial loads in which bearing deflection must be minimized.

Another double-row ball bearing is the internal self-aligning type, which is shown in Ill. 0-9.12. It compensates for angular misalignment, which can be caused by errors in mounting, shaft deflection, misalignment, etc. This bearing supports moderate radial loads and limited thrust loads.

Ill. 0-9.12 Double-row internal self-aligning bearing.

Ill. 0-9.13 Types of roller elements.

Roller: As with plain and ball bearings, roller bearings also may be classified by their ability to support radial, thrust, and combination loads. Note that combination load-supporting roller bearings are not called angular-contact bearings as they are with ball bearings. For example, the taper-roller bearing is a combination load-carrying bearing by virtue of the shape of its rollers.

Ill. 0-9.13 shows the different types of roller elements used in these bearings.

Roller elements are classified as cylindrical, barrel, spherical, and tapered. Note that barrel rollers are called needle rollers when less than 0.25-inch in diameter and have a relatively high ratio of length to diameter.

Cylindrical: Cylindrical bearings have solid or helically wound hollow cylindrically shaped rollers, which have an approximate length-diameter ratio ranging from 1:1 to 1:3. They normally are used for heavy radial loads beyond the capacities of comparably sized radial ball bearings.

Cylindrical bearings are especially useful for free axial movement of the shaft.

The free ring may have a restraining flange to provide some restraint to endwise movement in one direction. Another configuration comes without a flange, which allows the bearing rings to be displaced axially.

Either the rollers or the roller path on the races may be slightly crowned to prevent edge loading under slight shaft misalignment. Low friction makes this bearing type suitable for fairly high speeds. Ill. 0-9.14 shows a typical cylindrical roller bearing.

Ill. 0-9.14 Cylindrical roller bearing.

Ill. 0-9.15 Separable inner-ring-type cylindrical roller bearings.

Ill. 0-9.16 Separable inner-ring-type cylindrical roller bearings with different inner ring. Ill. 0-9.15 shows separable inner-ring cylindrical roller bearings. Ill. 0-9.16

Ill. 0-9.17 Separable inner-ring-type cylindrical roller bearings with elimination of a retainer ring on one side.

Ill. 0-9.18 Needle bearing, shows separable inner-ring cylindrical roller bearings with a different inner ring.

The roller assembly in Ill. 0-9.15 is located in the outer ring with retaining rings. The inner ring can be omitted and the roller operated on hardened ground shaft surfaces.

The style in Ill. 0-9.16 is similar to the one in Ill. 0-9.15, except the rib on the inner ring is different. This prohibits the outer ring from moving in a direction toward the rib.

Ill. 0-9.17 shows separable inner-ring-type cylindrical roller bearings with elimination of a retainer ring on one side.

The style shown in Ill. 0-9.17 is similar to the two previous styles except for the elimination of a retainer ring on one side. It can carry small thrust loads in only one direction.

Needle-Type Cylindrical or Barrel Needle-type cylindrical bearings ( Ill. 0-9.18) incorporate rollers that are symmetrical with a length at least four times their diameter. They are sometimes referred to as barrel roller s. These bearings are most useful where space is limited and thrust-load support is not required. They are available with or without an inner race. If a shaft takes the place of an inner race, it must be hardened and ground. The full-complement type is used for high loads and oscillating or slow speeds. The cage type should be used for rotational motion.

They come in both single-row and double-row mountings. As with all cylindrical roller bearings, the single-row mounting type has a low thrust capacity, but angular mounting of rolls in the double-row type permits its use for combined axial and thrust loads.

Ill. 0-9.19 Spherical roller bearing assembly.

Spherical : Spherical bearings are usually furnished in a double-row mounting that's inherently self-aligning. Both rows of rollers have a common spherical outer raceway. The rollers are barrel-shaped with one end smaller to provide a small thrust to keep the rollers in contact with the center guide flange.

This type of roller bearing has a high radial and moderate-to-heavy thrust load-carrying capacity. It maintains this capability with some degree of shaft and bearing housing misalignment. While their internal self-aligning feature is useful, care should be taken in specifying this type of bearing to compensate for misalignment. Ill. 0-9.19 shows a typical spherical roller bearing assembly.

Ill. 0-9.20 shows a series of spherical roller bearings for a given shaft size.

Tapered: Tapered bearings are used for heavy radial and thrust loads. They have straight tapered rollers, which are held in accurate alignment by means of a guide flange on the inner ring. Ill. 0-9.21 shows a typical tapered-roller bearing.

Ill. 0-9.22 shows necessary information to identify a taper-roller bearing. Ill. 0-9.23 shows various types of tapered roller bearings.

True rolling occurs because they are designed so that all elements in the rolling surface and the raceways intersect at a common point on the axis. The basic characteristic of these bearings is that if the apexes of the tapered working surfaces of both rollers and races were extended, they would coincide on the bearing axis. Where maximum system rigidity is required, they can be adjusted for a preload. These bearings are separable.

Ill. 0-9.20 Series of spherical roller bearings for a given shaft size Ill. 0-9.21 Tapered roller bearing.

Ill. 0-9.22 Information needed to identify a tapered roller bearing.

BEARING MATERIALS

Because two contacting metal surfaces are in motion in bearing applications, material selection plays a crucial role in their life. Properties of the materials used in bearing construction determine the amount of sliding friction that occurs, a key factor affecting bearing life. When two similar metals are in contact without the presence of adequate lubrication, friction is generally high and the surfaces will seize (i.e., weld) at relatively low pressures or surface loads. How-ever, certain combinations of materials support substantial loads without seizing or welding as a result of their low frictional qualities.

In most machinery, shafts are made of steel. Bearings are generally made of softer materials that have low frictional as well as sacrificial qualities when in contact with steel. A softer, sacrificial material is used for bearings because it's easier and cheaper to replace a worn bearing as opposed to a worn shaft.

Common bearing materials are cast iron, bronze, and babbitt. Other less commonly used materials include wood, plastics, and other synthetics.

There are several important characteristics to consider when specifying bearing materials, including the following: (1) strength or ability to withstand loads without plastic deformation; (2) ability to permit embedding of grit or dirt particles that are present in the lubricant; (3) ability to elastically deform to permit load distribution over the full bearing surface; (4) ability to dissipate heat and prevent hot spots that might seize; and (5) corrosion resistance.

Ill. 0-9.23 Various types of tapered roller bearings.

PLAIN

As indicated above, dissimilar metals with low frictional characteristics are most suitable for plain bearing applications. With steel shafts, plain bearings made of bronze or babbitt are commonly used. Bronze is one of the harder bearing materials and is generally used for low speeds and heavy loads.

A plain bearing may sometimes be made of a combination of materials. The outer portion may be constructed of bronze, steel, or iron to provide the strength needed to provide a load-carrying capability. The bearing may be lined with a softer material such as babbitt to provide the sacrificial capability needed to protect the shaft.

ROLLING ELEMENT

A specially developed steel alloy is used for an estimated 98% of all rolling element bearing uses. In certain special applications, however, materials such as glass, plastic, and other substances are sometimes used in rolling element construction.

Bearing steel is a high-carbon chrome alloy with high harden-ability and good toughness characteristics in the hardened and drawn state. All load-carrying members of most rolling contact bearings are made with this steel.

Controlled procedures and practices are necessary to ensure specification of the proper alloy, maintain material cleanliness, and ensure freedom from defects-all of which affect bearing reliability. Alloying practices that conform to rigid specifications are required to reduce anomalies and inclusions that adversely affect a bearing's useful life. Magnaflux inspections ensure that rolling elements are free from material defects and cracks. Light etching is used between rough and finish grinding processes to stop burning during heavy machining operations.

LUBRICATION

It is critical to consider lubrication requirements when specifying bearings.

Factors affecting lubricants include relatively high speeds, difficulty in performing re-lubrication, non-horizontal shafts, and applications where leakage can't be tolerated. This section briefly discusses lubrication mechanisms and techniques for bearings.

PLAIN BEARINGS

In plain bearings, the lubricating fluid must be replenished to compensate for end leakage to maintain their load-carrying capacity. Pressure lubrication from a pump- or gravity-fed tank, or automatic lubricating devices such as oil rings or oil disks, are provided in self-contained bearings. Another means of lubrication is to submerge the bearing (in particular, thrust bearings for vertical shafts) in an oil bath.

Lubricating Fluids

Almost any process fluid may be used to lubricate plain bearings if parameters such as viscosity, corrosive action, toxicity, change in state (where a liquid is close to its boiling point), and (in the case of a gaseous fluid) compressibility are appropriate for the application. Fluid-film journal and thrust bearings have run successfully, for example, on water, kerosene, gasoline, acid, liquid refrigerants, mercury, molten metals, and a wide variety of gases.

Gases, however, lack the cooling and boundary-lubrication capabilities of most liquid lubricants. Therefore the operation of self-acting gas bearings is restricted by start/stop friction and wear. If start/stop is performed under load, then the design is limited to about 7 pounds per square inch (l b = i n 2 ) or 48 kilo-Newtons per square meter (k N m^2 ) on the projected bearing area, depending on the choice of materials. In general, the materials used for these bearings are those of dry rubbing bearings (e.g., either a hard/hard combination such as ceramics with or without a molecular layer of boundary lubricant or a hard/soft combination with a plastic surface).

Externally pressurized gas journal bearings have the same principle of operation as hydrostatic liquid-lubricated bearings. Any clear gas can be used, but many of the design charts are based on air. There are three forms of external flow restrictors in use with these bearings: pocketed (simple) orifice, unpocketed (annular) orifice, and slot.

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Tbl. 0-9.9 Ball-Bearing Grease Re-lubrication Intervals (Hours of Operation)

Bearing Bore, mm Bearing Speed, rpm 5,000 3,600 1,750 1,000 200

10 8,700 12,000 25,000 44,000 220,000 20 5,500 8,000 17,000 30,000 150,000 30 4,000 6,000 13,000 24,000 127,000 40 2,800 4,500 11,000 20,000 111,000 50 3,500 9,300 18,000 97,000 60 2,600 8,000 16,000 88,000 70 6,700 14,000 81,000 80 5,700 12,000 75,000 90 4,800 11,000 70,000 100 4,000 10,000 66,000

= =

Tbl. 0-9.10 Oil Lubrication Viscosity (ISO Identification Numbers)

Bearing Bore, mm Bearing Speed, rpm 10,000 3,600 1,800 600 50

4-7 68 150 220 10-20 32 68 150 220 460 25-45 10 32 68 150 320 50-70 7 22 68 150 320 75-90 3 10 22 68 220 100 3 7 22 68 220

= =

State of Lubrication

Fluid or complete lubrication, the condition in which the surfaces are completely separated by a fluid film, provides the lowest friction losses and prevents wear.

The semi-fluid lubrication state exists between the journal and bearing when a load-carrying fluid film does not form to separate the surfaces. This occurs at comparatively low speed with intermittent or oscillating motion, heavy load, and insufficient oil supply to the bearing. Semi-fluid lubrication also may exist in thrust bearings with fixed parallel-thrust collars; guide bearings of machine tools; bearings with plenty of lubrication, but a bent or misaligned shaft; or where the bearing surface has improperly arranged oil grooves. The coefficient of friction in such bearings may range from 0.02 to 0.08.

In situations where the bearing is well lubricated but the speed of rotation is very slow or the bearing is barely greasy, boundary lubrication takes place. In this situation, which occurs in bearings when the shaft is starting from rest, the coefficient of friction may vary from 0.08 to 0.14.

A bearing may run completely dry in exceptional cases of design or with a complete failure of lubrication. Depending on the contacting surface materials, the coefficient of friction will be between 0.25 and 0.40.

ROLLING ELEMENT BEARINGS

Rolling element bearings also need a lubricant to meet or exceed their rated life.

In the absence of high temperatures, however, excellent performance can be obtained with a very small quantity of lubricant. Excess lubricant causes excessive heating, which accelerates lubricant deterioration.

The most popular type of lubrication is the sealed grease ball-bearing cartridge.

Grease is commonly used for lubrication because of its convenience and minimum maintenance requirements. A high-quality lithium-based NLGI 2 grease is commonly used for temperatures up to 18 0 8F (8 2 8C). Grease must be replenished and re-lubrication intervals in hours of operation are dependent on temperature, speed, and bearing size. Tbl. 0-9.9 is a general guide to the time after which it's advisable to add a small amount of grease.

Some applications, however, can't use the cartridge design-for example, when the operating environment is too hot for the seals. Another example is when minute leaks or the accumulation of traces of dirt at the lip seals can't be tolerated (e.g., food processing machines). In these cases, bearings with specialized sealing and lubrication systems must be used.

In applications involving high speed, oil lubrication is typically required. Tbl. 0-9.10 is a general guide in selecting oil of the proper viscosity for these bearings.

For applications involving high-speed shafts, bearing selection must take into account the inherent speed limitations of certain bearing designs, cooling needs, and lubrication issues such as churning and aeration suppression. A typical case is the effect of cage design and roller-end thrust-flange contact on the lubrication requirements in tapered roller bearings. These design elements limit the speed and the thrust load that these bearings can endure. As a result, it's important to always refer to the bearing manufacturer's instructions on load-carrying design and lubrication specifications.

INSTALLATION and GENERAL HANDLING PRECAUTIONS

Proper handling and installation practices are crucial to optimal bearing performance and life. In addition to standard handling and installation practices, the issue of emergency bearing substitutions is an area of critical importance. If substitute bearings are used as an emergency means of getting a machine back into production quickly, the substitution should be entered into the historical records for that machine. This documents the temporary change and avoids the possibility of the substitute bearing becoming a permanent replacement. This error can be extremely costly, particularly if the incorrectly specified bearing continually fails prematurely. It is important that an inferior substitute be removed as soon as possible and replaced with the originally specified bearing.

PLAIN BEARING INSTALLATION

It is important to keep plain bearings from shifting sideways during installation and to ensure an axial position that does not interfere with shaft fillets. Both of these can be accomplished with a locating lug at the parting line. Less frequently used is a dowel in the housing, which protrudes partially into a mating hole in the bearing.

The distance across the outside parting edges of a plain bearing are manufactured slightly greater than the housing bore diameter. During installation, a light force is necessary to snap it into place and , once installed, the bearing stays in place because of the pressure against the housing bore.

It is necessary to prevent a bearing from spinning during operation, which can cause a catastrophic failure. Spinning is prevented by what is referred to as ''crush.'' Bearings are slightly longer circumferentially than their mating housings, and on installation, this excess length is elastically deformed or ''crushed.'' This sets up a high radial contact pressure between the bearing and housing, which ensures good back contact for heat conduction and , in combination with the bore-to bearing friction, prevents spinning. It is important that under no circumstances should the bearing parting lines be filed or otherwise altered to remove the crush.

ROLLER BEARING INSTALLATION

A basic rule of rolling element bearing installation is that one ring must be mounted on its mating shaft or in its housing with an interference fit to prevent rotation. This is necessary because it's virtually impossible to prevent rotation by clamping the ring axially.

Mounting Hardware

Bearings come as separate parts that require mounting hardware or as pre-mounted units that are supplied with their own housings, adapters, and seals.

Bearing Mountings

Typical bearing mountings, which are shown in Ill. 0-9.24, locate and hold the shaft axially and allow for thermal expansion and /or contraction of the shaft.

Locating and holding the shaft axially is generally accomplished by clamping one of the bearings on the shaft so that all machine parts remain in proper relation-ship dimensionally. The inner ring is locked axially relative to the shaft by locating it between a shaft shoulder and some type of removable locking device once the inner ring has a tight fit. Typical removable locking devices are specially designed nuts, which are used for a through shaft, and clamp plates, which are commonly used when the bearing is mounted on the end of the shaft. For the locating or held bearing, the outer ring is clamped axially, usually between housing shoulders or end-cap pilots.

With general types of cylindrical roller bearings, shaft expansion is absorbed internally simply by allowing one ring to move relative to the other ( Ill. 0-9.24a and 9.24c, non-locating positions). The advantage of this type of mounting is that both inner and outer rings may have a tight fit, which is desirable or even mandatory if significant vibration and /or imbalance exists in addition to the applied load.

Ill. 0-9.24 Typical bearing mounting.

Ill. 0-9.25 Typical pillow block.

Ill. 0-9.26 Flanged bearing unit.

Pre-mounted Bearing

Pre-mounted bearings, referred to as pillow-block and flanged-housing mountings, are of considerable importance to millwrights. They are particularly adaptable to

''line-shafting'' applications, which are a series of ball and roller bearings sup-plied with their own housings, adapters, and seals. Pre-mounted bearings come with a wide variety of flange mountings, which permit them to be located on faces parallel or perpendicular to the shaft axis. Ill. 0-9.25 shows a typical pillow block. Ill. 0-9.26 shows a flanged bearing unit.

Inner races can be mounted directly on ground shafts or can be adapter-mounted to ''drill-rod'' or to commercial shafting. For installations sensitive to imbalance and vibration, the use of accurately ground shaft seats is recommended.

Most pillow-block designs incorporate self-aligning bearing types and don't require the precision mountings utilized with other bearing installations.

Mounting Techniques

When mounting or dismounting a roller bearing, the most important thing to remember is to apply the mounting or dismounting force to the side face of the ring with the interference fit. This force should not pass from one ring to the other through the ball or roller set, because internal damage can easily occur.

Mounting tapered-bore bearings can be accomplished simply by tightening the locknut or clamping plate. This locates it on the shaft until the bearing is forced the proper distance up the taper. This technique requires a significant amount of force, particularly for large bearings.

Cold Mounting

Cold mounting, or force-fitting a bearing onto a shaft or into a housing, is appropriate for all small bearings (i.e., 4-inch bore and smaller). The force, however, must be applied as uniformly as possible around the side face of the bearing and to the ring to be press-fit. Mounting fixtures, such as a simple piece of tubing of appropriate size and a flat plate, should be used. It is not appropriate to use a drift and hammer to force the bearing on, which will cause the bearing to cock. It is possible to apply force by striking the plate with a hammer or by an arbor press. However, before forcing the bearing on the shaft, a coat of light oil should be applied to the bearing seat on the shaft and the bearing bores. All sealed and shielded ball bearings should be cold mounted in this manner.

Temperature Mounting

The simplest way to mount any open straight-bore bearing regardless of its size is temperature mounting, which entails heating the entire bearing, pushing it on its seat, and holding it in place until it cools enough to grip the shaft. The housing may be heated if practical for tight outside-diameter fits; however, temperatures should not exceed 25 0 8F. If heating of the housing is not practical, the bearing may be cooled with dry ice. The risk of cooling is that if the ambient conditions are humid, moisture is introduced and there is a potential for corrosion in the future. Acceptable ways of heating bearings are by hot plate, temperature-controlled oven, induction heaters, and hot-oil bath.

With the hot plate method, the bearing is simply laid on the plate until it reaches the approved temperature, with a pyrometer or Tempilstik used to make certain it's not overheated. Difficulty in controlling the temperature is the major disadvantage of this method.

When using a temperature-controlled oven, the bearings should be left in the oven long enough to heat thoroughly, but they should never be left overnight.

The use of induction heaters is a quick method of heating bearings. However, some method of measuring the ring temperature (e.g., pyrometer or a Tempilstik) must be used or damage to the bearing may occur. Note that bearings must be demagnetized after the use of this method.

The use of a hot-oil bath is the most practical means of heating larger bearings.

Disadvantages are that the temperature of the oil is hard to control, and it may ignite or overheat the bearing. The use of a soluble oil-and-water mixture (10-15% oil) can eliminate these problems and still attain a boiling temperature of 21 0 8F. The bearing should be kept off the bottom of the container by a grate or screen located several inches off the bottom. This is important to allow contaminants to sink to the bottom of the container and away from the bearing.

Ill. 0-9.27 Ball installation procedures.

[ 2. Balls are installed in the gap. 4. A retainer in installed. 1. The inner ring is moved to one side. 3. The inner ring is centered as the balls are equally positioned in place.]

Dismounting

Commercially available bearing pullers allow rolling element bearings to be dismounted from their seats without damage. When removing a bearing, force should be applied to the ring with the tight fit, although sometimes it's necessary to use supplementary plates or fixtures. An arbor press is equally effective at removing smaller bearings as well as mounting them.

Ball Installation

Ill. 0-9.27 shows the ball installation procedure for roller bearings. The designed load carrying capacity of Conrad-type bearings is determined by the number of balls that can be installed between the rings. Ball installation is accomplished by the following procedure:

_ Slip the inner ring slightly to one side

_ Insert balls into the gap, which centers the inner ring as the balls are positioned between the rings

_ Place stamped retainer rings on either side of the balls before riveting together. This positions the balls equidistant around the bearing.

GENERAL ROLLER-ELEMENT BEARING HANDLING PRECAUTIONS

For roller-element bearings to achieve their design life and perform with no abnormal noise, temperature rise, or shaft excursions, the following precautions should be taken:

_ Always select the best bearing design for the application and not the cheapest. The cost of the original bearing is usually small by comparison to the costs of replacement components and the down-time in production when premature bearing failure occurs because an inappropriate bearing was used.

_ If in doubt about bearings and their uses, consult the manufacturer's representative and the product literature.

_ Bearings should always be handled with great care. Never ignore the handling and installation instructions from the manufacturer.

_ Always work with clean hands, clean tools, and the cleanest environment available.

_ Never wash or wipe bearings prior to installation unless the instructions specifically state that this should be done. Exceptions to this rule are when oil-mist lubrication is to be used and the slushing compound has hardened in storage or is blocking lubrication holes in the bearing rings. In this situation, it's best to clean the bearing with kerosene or other appropriate petroleum-based solvent. The other exception is if the slushing compound has been contaminated with dirt or foreign matter before mounting.

_ Keep new bearings in their greased paper wrappings until they are ready to install. Place unwrapped bearings on clean paper or lint-free cloth if they can't be kept in their original containers. Wrap bearings in clean, oil-proof paper when not in use.

_ Never use wooden mallets, brittle or chipped tools, or dirty fixtures and tools when bearings are being installed.

_ Do not spin bearings (particularly dirty ones) with compressed service air.

_ Avoid scratching or nicking bearing surfaces. Care must be taken when polishing bearings with emery cloth to avoid scratching.

_ Never strike or press on race flanges.

_ Always use adapters for mounting that ensure uniform steady pressure rather than hammering on a drift or sleeve. Never use brass or bronze drifts to install bearings as these materials chip very easily into minute particles that will quickly damage a bearing.

_ Avoid cocking bearings onto shafts during installation.

_ Always inspect the mounting surface on the shaft and housing to ensure that there are no burrs or defects.

_ When bearings are being removed, clean housings and shafts before exposing the bearings. Dirt is abrasive and detrimental to the designed life span of bearings.

_ Always treat used bearings as if they are new, especially if they are to be reused.

_ Protect dismantled bearings from moisture and dirt.

_ Use clean, filtered, water-free Stoddard's solvent or flushing oil to clean bearings.

_ When heating is used to mount bearings onto shafts, follow the manufacturer's instructions.

_ When assembling and mounting bearings onto shafts, never strike the outer race or press on it to force the inner race. Apply the pressure on the inner race only. When dismantling, follow the same procedure.

_ Never press, strike, or otherwise force the seal or shield on factory-sealed bearings.

BEARING FAILURES, DEFICIENCIES, and THEIR CAUSES

The general classifications of failures and deficiencies requiring bearing removal are overheating, vibration, turning on the shaft, binding of the shaft, noise during operation, and lubricant leakage. Tbl. 0-9.11 is a troubleshooting guide that lists the common causes for each of these failures and deficiencies. As indicated by the causes of failure listed, bearing failures are rarely caused by the bearing itself.

Many abnormal vibrations generated by actual bearing problems are the result of improper sizing of the bearing liner or improper lubrication. However, numerous machine and process-related problems generate abnormal vibration spectra in bearing data. The primary contributors to abnormal bearing signatures are (1) imbalance, (2) misalignment, (3) rotor instability, (4) excessive or abnormal loads, and (5) mechanical looseness.

Defective bearings that leave the manufacturer are very rare, and it's estimated that defective bearings contribute to only 2% of total failures. The failure is invariably linked to symptoms of misalignment, imbalance, resonance, and lubrication-or the lack of it. Most of the problems that occur result from the following reasons: dirt, shipping damage, storage and handling, poor fit resulting in installation damage, wrong type of bearing design, overloading, improper lubrication practices, misalignment, bent shaft, imbalance, resonance, and soft foot. Any one of these conditions will eventually destroy a bearing-two or more of these problems can result in disaster! Although most industrial machine designers provide adequate bearings for their equipment, there are some cases where bearings are improperly designed, manufactured, or installed at the factory. Usually, however, the trouble is caused by one or more of the following reasons: (1) improper on-site bearing selection and / or installation, (2) incorrect grooving, (3) unsuitable surface finish, (4) insufficient clearance, (5) faulty relining practices, (6) operating conditions, (7) excessive operating temperature, (8) contaminated oil supply, and (9) oil-film instability.

IMPROPER BEARING SELECTION AN D/ OR INSTALLATION

There are several things to consider when selecting and installing bearings, including the issue of interchangeability, materials of construction, and damage that might have occurred during shipping, storage, and handling.

Interchangeability

Because of the standardization in envelope dimensions, precision bearings were once regarded as interchangeable among manufacturers. This interchangeability has since been considered a major cause of failures in machinery, and the practice should be used with extreme caution.

Most of the problems with interchangeability stem from selecting and replacing bearings based only on bore size and outside diameters. Often, very little consideration is paid to the number of rolling elements contained in the bearings.

This can seriously affect the operational frequency vibrations of the bearing and may generate destructive resonance in the host machine or adjacent machines.

More bearings are destroyed during their installation than fail in operation. Installation with a heavy hammer is the usual method in many plants. Heating the bearing with an oxyacetylene burner is another classical method. However, the bearing does not stand a chance of reaching its life expectancy when either of these installation practices is used. The bearing manufacturer's installation instructions should always be followed.

Materials of Construction

Refer to Section 3, which discusses the appropriate materials of construction for the different types of bearings.

Shipping Damage

Bearings and the machinery containing them should be properly packaged to avoid damage during shipping. However, many installed bearings are exposed to vibration, bending, and massive shock loading through bad handling practices during shipping. It has been estimated that approximately 40% of newly received machines have ''bad'' bearings.

Because of this, all new machinery should be thoroughly inspected for defects before installation. Acceptance criteria should include guidelines that clearly define acceptable design/operational specifications. This practice pays big dividends by increasing productivity and decreasing unscheduled downtime.

Storage and Handling

Stores and other appropriate personnel must be made aware of the potential havoc they can cause by their mishandling of bearings. Bearing failure often starts in the storeroom rather than in the machinery. Premature opening of packages containing bearings should be avoided whenever possible. If packages must be opened for inspection, they should be protected from exposure to harmful dirt sources and then resealed in the original wrappings. The bearing should never be dropped or bumped as this can cause shock loading on the bearing surface.

Incorrect Placement of Oil Grooves

Incorrectly placed oil grooves can cause bearing failure. Locating the grooves in high-pressure areas causes them to act as pressure-relief passages. This interferes with the formation of the hydrodynamic film, resulting in reduced load-carrying capability.

Unsuitable Surface Finish

Smooth surface finishes on both the shaft and the bearing are important to prevent surface variations from penetrating the oil film. Rough surfaces can cause scoring, overheating, and bearing failure. The smoother the finishes, the closer the shaft may approach the bearing without danger of surface contact.

Although important in all bearing applications, surface finish is critical with the use of harder bearing materials such as bronze.

Insufficient Clearance

There must be sufficient clearance between the journal and bearing to allow an oil film to form. An average diameter clearance of 0.001 in. per inch of shaft diameter is often used. This value may be adjusted depending on the type of bearing material, the load, speed, and the accuracy of the shaft position desired.

Faulty Relining

Faulty relining occurs primarily with babbitted bearings rather than precision machine-made inserts. Babbitted bearings are fabricated by a pouring process that should be performed under carefully controlled conditions. Some reasons for faulty relining are (1) improper preparation of the bonding surface, (2) poor pouring technique, (3) contamination of babbitt, and (4) pouring bearing to size with journal in place.

Operating Conditions

Abnormal operating conditions or neglecting necessary maintenance precautions cause most bearing failures. Bearings may experience premature and /or catastrophic failure on machines that are operated heavily loaded, speeded up, or being used for a purpose not appropriate for the system design. Improper use of lubricants can also result in bearing failure. Some typical causes of premature failure include (1) excessive operating temperatures, (2) foreign material in the lubricant supply, (3) corrosion, (4) material fatigue, and (5) use of unsuitable lubricants.

Excessive Temperatures

Excessive temperatures affect the strength, hardness, and life of bearing materials.

Lower temperatures are required for thick babbitt liners than for thin precision babbitt inserts. Not only do high temperatures affect bearing materials, they also reduce the viscosity of the lubricant and affect the thickness of the film, which affects the bearing's load-carrying capacity. In addition, high temperatures result in more rapid oxidation of the lubricating oil, which can result in unsatisfactory performance.

Dirt and Contamination in Oil Supply

Dirt is one of the biggest culprits in the demise of bearings. Dirt makes its appearance in bearings in many subtle ways, and it can be introduced by bad work habits. It also can be introduced through lubricants that have been exposed to dirt, which is responsible for approximately half of bearing failures through-out the industry.

To combat this problem, soft materials such as babbitt are used when it's known that a bearing will be exposed to abrasive materials. Babbitt metal embeds hard particles, which protects the shaft against abrasion. When harder materials are used in the presence of abrasives, scoring and galling occurs as a result of abrasives caught between the journal and bearing.

In addition to the use of softer bearing materials for applications in which abrasives may potentially be present, it's important to properly maintain filters and breathers, which should regularly be examined. To avoid oil supply contamination, foreign material that collects at the bottom of the bearing sump should be removed on a regular basis.

Oil Film Instability

The primary vibration frequency components associated with fluid-film bearing problems are in fact displays of turbulent or non-uniform oil film. Such instabil-ity problems are classified as either oil whirl or oil whip, depending on the severity of the instability.

Machine-trains that use sleeve bearings are designed based on the assumption that rotating elements and shafts operate in a balanced and therefore centered position. Under this assumption, the machine-train shaft will operate with an even, concentric oil film between the shaft and sleeve bearing.

For a normal machine, this assumption is valid after the rotating element has achieved equilibrium. When the forces associated with rotation are in balance, the rotating element will center the shaft within the bearing. However, several problems directly affect this self-centering operation. First, the machine-train must be at designed operating speed and load to achieve equilibrium. Second, any imbalance or abnormal operation limits the machine-train's ability to center itself within the bearing.

A typical example is a steam turbine. A turbine must be supported by auxiliary running gear during start up or shut down to prevent damage to the sleeve bearings. The lower speeds during the start up and shut down phase of operation prevent the self-centering ability of the rotating element. Once the turbine has achieved full speed and load, the rotating element and shaft should operate without assistance in the center of the sleeve bearings.

Ill. 0-9.28 Oil whirl, oil whip.

Oil Whirl

In an abnormal mode of operation, the rotating shaft may not hold the centerline of the sleeve bearing. When this happens, an instability called oil whirl occurs. Oil whirl is an imbalance in the hydraulic forces within a sleeve bearing. Under normal operation, the hydraulic forces such as velocity and pressure are balanced.

If the rotating shaft is offset from the true centerline of the bearing, instability occurs.

As Ill. 0-9.28 illustrates, a restriction is created by the offset. This restriction creates a high pressure and another force vector in the direction of rotation. Oil whirl accelerates the wear and failure of the bearing and bearing support structure.

Oil Whip

The most severe damage results if the oil whirl is allowed to degrade into oil whip. Oil whip occurs when the clearance between the rotating shaft and sleeve bearing is allowed to close to a point approaching actual metal-to-metal contact.

When the clearance between the shaft and bearing approaches contact, the oil film is no longer free to flow between the shaft and bearing. As a result, the oil film is forced to change directions. When this occurs, the high-pressure area created in the region behind the shaft is greatly increased. This vortex of oil increases the abnormal force vector created by the offset and rotational force to the point that metal-to-metal contact between the shaft and bearing occurs. In almost all instances where oil whip is allowed, severe damage to the sleeve bearing occurs.

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